1. Field of the Invention
This invention relates to metal-polymer laminates and to their preparation. More particularly, this invention relates to novel metal-polymer-metal structural laminates having a unique combination of property advantages.
2. Description of the Prior Art
Metal-plastic-metal laminates have been described in various U.S. and foreign patents. Exemplary patents include U.S. Pat. No. 3,582,427, U.S. Pat. No. 4,229,504, U.S. Pat. No. 4,204,022, U.S. Pat. No. 4,313,996, U.S. Pat. No. 4,369,222 and EPA 19,835. These laminates are useful as light weight replacements for sheet steel in cars and trucks. Relatively thin laminates are useful in flexible packaging end use applications while relatively thick laminates are useful as construction laminates.
Methods of preparing such laminates are also known. One method includes bringing at least one layer of plastic and at least one layer of metal into intimate contact and subjecting them to suitable heat and pressure, using, for example, a platen press. A more efficient and continuous method involves the well known extrusion processes--extrusion coating or extrusion lamination. Often an intermediate layer of adhesive or primer, in the form of a film or coating, is used in conjunction with these methods in insure adequate adhesion between the metal substrate and the plastic.
In the past, one primary incentive for considering the replacement of sheet steel with metal-polymer laminates was the weight saving that could be obtained with equivalent stiffness. Placing thin steel skins on the outside of the laminate optimal use of high yield, high modulus steel and allows the structurally ineffective (in bending) middle portion of the composite to be light weight plastic, resulting in the primary advantage of steel-plastic laminates--weight reduction versus an equivalent stiffness sheet steel, but at substantially less cost penalty compared to other weight-reducing materials such as aluminum sheet. In other cases it has been desired to obtain sound or vibration damping from the laminate. In the past, in order to obtain such vibration damping, manufacturers would provide a laminate having relatively thick skins (15 or 20 mils) and a relatively thin, low modulus viscoelastic polymer core (3 to 6 mils). However, in order to obtain equivalent stiffness to the steel it replaced, it was necessary to increase the overall thickness of the steel in the sound damping laminate. This resulted in a much heavier laminate than the equivalent stiffness steel it replaced. What is needed are laminates that provide both light weight and sound damping.